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Bottlehead Kits => Crack => Topic started by: arkrazor on April 06, 2015, 07:00:39 PM

Title: Channel imbalance issues
Post by: arkrazor on April 06, 2015, 07:00:39 PM
Hey everyone, just finished a crack+speedball build not too long ago and everything works perfectly for the most part. The only problem that I'm having with my crack is that there's a very slight channel imbalance, with the left being more prominent than the right (or the right being weaker than the left).

All of my measurements are within acceptable ranges and I'm using the volume pot past the 11 mark. The only thing I think is worth noting though is that the volume pot's right channel is about 7k Ohms higher than the left channel and that Terminal's 1 and 5 have a roughly 7V difference. Other than that, everything measures in the green.

I've tried touching up the solder joints and swapped out the RCA jacks, anything else I should try out? I was thinking of swapping out the volume pot next, but I figured I should ask the experts before throwing down more dosh.

Thanks in advance!
Title: Re: Channel imbalance issues
Post by: JamieMcC on April 06, 2015, 10:01:30 PM
Hey everyone, just finished a crack+speedball build not too long ago and everything works perfectly for the most part. The only problem that I'm having with my crack is that there's a very slight channel imbalance, with the left being more prominent than the right (or the right being weaker than the left).

All of my measurements are within acceptable ranges and I'm using the volume pot past the 11 mark. The only thing I think is worth noting though is that the volume pot's right channel is about 7k Ohms higher than the left channel and that Terminal's 1 and 5 have a roughly 7V difference. Other than that, everything measures in the green.

I've tried touching up the solder joints and swapped out the RCA jacks, anything else I should try out? I was thinking of swapping out the volume pot next, but I figured I should ask the experts before throwing down more dosh.

Thanks in advance!

In the sticky section of the Crack there is a FAQ where a solution for this is given.

http://bottlehead.com/smf/index.php?board=24.0

Title: Re: Channel imbalance issues
Post by: Grainger49 on April 07, 2015, 02:09:44 AM
If it is an imbalance throughout the volume range it is probably a tube. 

If you can not use most of the volume range try what Jamie suggests.  It is always much better being able to use all the volume control.  It gives you finer adjustments.
Title: Re: Channel imbalance issues
Post by: arkrazor on April 07, 2015, 07:20:03 AM
Hey everyone, thanks for the input.

I forgot to also mention that I already did the resistor mod as detailed in the crack FAQ. The channel imbalance persists throughout the entire range of the volume pot. I also thought that it might've been a tube problem so I ordered a used set off of eBay. No luck with that either. Could the seller have given me a bad tube or is it something with my circuit? Should I post my voltage readings just in case?
Title: Re: Channel imbalance issues
Post by: Paul Joppa on April 07, 2015, 10:54:36 AM
Was the channel balance problem there before the resistor mod? If not, then it's worth checking that the right resistors are in the right places, and that the various resistances (pot output, pot input, RCA jack) are matched between channels. Set gain to the usual, I think that was halfway up?

I suggest the above because since the tube swap had no effect, the input wiring becomes a suspect.  :^)
Title: Re: Channel imbalance issues
Post by: arkrazor on April 07, 2015, 01:05:43 PM
Hey Paul, appreciate the input.

I double checked and the resistance imbalance is most likely originating from the volume pot. Here are the readings for what they're worth.

R RCA jack: 204K Ohms
L RCA jack: 197K Ohms-198K Ohms
R Input Pot: 97K Ohms
L Input Pot: 90.3K Ohms
R Output Pot at half gain: 12.3K Ohms
L Output Pot at half gain: 15K Ohms

At full gain, the output resistance matches the input resistance.

Is a 3K-7K Ohm difference enough to make a noticeable channel imbalance?
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on April 07, 2015, 02:41:34 PM

Is a 3K-7K Ohm difference enough to make a noticeable channel imbalance?
It's a far more complicated question than that.  Based on your numbers, you just have 100K padding resistors before the pot? 

At full volume, the right half will have -6.45dB of attenuation, while the left half will be -6.77.  It's safe to say that this is inaudible.

Can you download a 60Hz sine wave and play it into your amp with your phone?  If so, let me know, and we can trace signal through your amp and see where it's getting lost.

-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on April 07, 2015, 06:56:06 PM
Hey Paul,

Correct, there is roughly 100k padding resistors before the pot. I have a 75k and a 33k resistor installed in each channel, as per the crack FAQ.

Unfortunately, I don't have a 3.5mm to RCA adapter, so I don't think I can use my phone to play the 60hz sine wave. Is using a computer out of the question?
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on April 08, 2015, 08:40:23 AM
A computer will work just fine. If you have the 33K and 75K resistors, you shouldn't have 200K from center lug to ground on the RCA jacks (it should still be 100K).  If you have the 33K resistors across the pot, your R/L input at the pot should show 24K.

If you'd like, you can post a picture of what you have wired.  Based on this information, I would say that the resistors are not correctly installed.

-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on April 10, 2015, 06:54:40 AM
Hey Paul,

Unfortunately, I am out of town at the moment, so I can't take a picture of my setup. I do remember how it was wired though. I had the two resistors put in between the wires leading from the RCA jacks to the volume pot itself. I'm guessing this configuration is incorrect. Do you, by any chance, happen to have a picture of the correct configuration?
Title: Re: Channel imbalance issues
Post by: kekoukela on April 22, 2015, 02:44:52 PM
I would like some pictures of the correct setup as well
Title: Re: Channel imbalance issues
Post by: arkrazor on June 05, 2015, 10:48:37 AM
Hey everyone, sorry for the late reply. Been busy with some irl priorities and whatnot.

Anyway, I took a look at my crack again and tried to see if I could fix it. I read through the instructions again, and idk if it's just my problem, but they seem really ambiguous. I want to take a picture, but all I have is my phone camera, which is absolute rubbish at taking close up photos. I'll try to describe my setup in better detail.

Right now, I have a 33k resistor between the input and output lugs on the volume pot, for both levels. Should there be another resistor connected to the ground? I also have the 75k resistors in between the RCA jack and the volume pot, but based on Paul's previous information, I'm assuming this is incorrect. Should the 75k resistors be between the output wires and the volume pot?

Here's a quick, poorly drawn diagram I whipped up for further clarification:
(http://i.imgur.com/YgMlZbW.png)

Is this the correct setup? The picture applies to both levels.
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 05, 2015, 04:41:34 PM
The 33K resistors go between input and ground on the pot.  The 75K resistors go from the input lugs on the pot to the wires coming from the RCA jacks.
Title: Re: Channel imbalance issues
Post by: arkrazor on June 16, 2015, 09:25:46 AM
Thanks for the input, Paul. I think I have everything in its correct place. What should I do with the 60hz sine wave now?
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 16, 2015, 11:17:05 AM
Thanks for the input, Paul. I think I have everything in its correct place. What should I do with the 60hz sine wave now?

Do you still have a channel imbalance?
Title: Re: Channel imbalance issues
Post by: arkrazor on June 18, 2015, 06:54:40 PM
Unfortunately, yes.
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 19, 2015, 07:14:04 AM
OK, start by playing a 60Hz tone into the RCA jacks.  Measure the AC voltage from ground to the center pin of each RCA jack and post those here.

-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on June 20, 2015, 06:34:59 AM
AC Voltages from Ground to the Center pin of each RCA jack measure at around 1.538V.
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 20, 2015, 01:35:37 PM
Next, turn the volume control all the way up and perform the same measurement at each of the center lugs of the volume pot.  (Amp still off)
Title: Re: Channel imbalance issues
Post by: arkrazor on June 21, 2015, 02:10:06 PM
I'm getting .375 for the top lug and .370 for the bottom lug
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 22, 2015, 06:50:55 AM
Just to be thorough, with the amp off, measure the AC voltage again at A2 and A7.  It should match what you measured previously.


If that checks out, then with the amp on, carefully measure the AC voltage at terminal 6 and terminal 10.  If you have the shorting modification at the headphone jack, let me know and I can give you some options for bypassing it.


-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on June 22, 2015, 07:31:02 AM
A2 and A7 voltages check out. Maybe + or - .001V

I do have the shorting mod for the headphone jack installed.
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 23, 2015, 11:07:48 AM
The next thing to do is to try measuring the AC voltage at terminals 1 and 5 with the amplifier on and the volume control all the way up.  Your meter may not be able to measure the AC voltage there with the DC voltage present, but it's worth a shot. 

-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on June 23, 2015, 07:36:42 PM
If I measured correctly, terminal 5 is at 7.68 and terminal 1 is at 7.73
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 24, 2015, 09:13:38 AM
Excellent, that's 0.05dB of difference between channels, which is difficult to measure, and impossibly hard to hear.

The next set of measurements should be done at terminals 6 and 10.

Since you have the shorting jack, these will both be 0V unless we take some action.  If you can find a pair of toothpicks, you can pop them in under the spring contacts for the tip and ring on the jack, just to lift the short.  Alternatively, if you have a 1/4" TRS to 1/8 TRS adapter plug, go ahead and plug that in to release the spring contacts. 

I would not use a pair of headphones to break this connection, as we want the signal level high and we want the headphones themselves out of the equation.

-PB
Title: Re: Channel imbalance issues
Post by: Strikkflypilot on June 24, 2015, 10:01:42 AM
Or attach headphone cables but detach at the cups if lacking an adapter?
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 24, 2015, 10:23:16 AM
Yeah, if the cables can be unplugged from the drivers (HD580/600/650), that may work.  There still may be a cabling issue, but it's a place to start.

-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on June 24, 2015, 02:17:48 PM
Hey guys, I managed to find a right angle 1/4" TRS to 1/4" TRS and it looked like it did the trick. Terminal 6 measures at 4.77 and Terminal 10 measures at 4.88
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 25, 2015, 08:21:06 AM
That's 0.2dB of difference, still not much.

With the adapter still plugged in, measure that voltage at the headphone jack (red and white wire terminals on the jack).

-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on June 25, 2015, 11:12:30 AM
I'm getting similar measurements. White terminal is measuring at 4.87; red terminal is measuring at 4.77. Nothing out of the ordinary, it seems.
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 25, 2015, 11:23:15 AM
Which headphones do you have?  If you have HD580/600/650, I would recommend unplugging your headphone cable from your headphones, plugging the cable into the amp, then measuring the AC voltage at the headphone end of your cables.

The good news is that your Crack has no channel imbalance to speak of.  The bad news is that it's either your headphones, source, headphone cables, or you still have an installation issue with the padding resistors that go on the pot.  (maybe post a pic?)

-PB
Title: Re: Channel imbalance issues
Post by: arkrazor on June 25, 2015, 09:03:56 PM
I have the HD650's. I just measured through the headphone cables and the measurements match the terminal's 6 and 10 measurements. I believe I installed the padding resistors correctly because the resistance measurements match those you previously said (~24k at the input for both). I attached some pics of it to the post (sorry for the rubbish quality).

I did an AB test with my solid state amp and the crack and I swear I can still hear channel imbalance. Could it be caused by the tubes? I only have the stock ones and another set I bought second hand off of eBay. I won't rule out the possibility that I might've gotten unlucky twice in a row. It's either that or I need to get my hearing checked.
Title: Re: Channel imbalance issues
Post by: Paul Birkeland on June 28, 2015, 09:00:22 AM
I have the HD650's. I just measured through the headphone cables and the measurements match the terminal's 6 and 10 measurements.
Well, that confirms that there is a channel imbalance, but it's about 10X too small to be audible.  You may want to consider checking the channel balance of your solid state amp.

-PB
Title: Re: Channel imbalance issues
Post by: fullheadofnothing on June 28, 2015, 02:20:15 PM
Furthermore, are you hearing this imbalance on the 60Hz tone, or are you just listening to stereo recordings that will present different channel information?
Title: Re: Channel imbalance issues
Post by: Paul Joppa on June 28, 2015, 02:58:22 PM
Just checking here, without re-reading the whole thread ... but you did turn the headphone around to see if the imbalance moved to the other ear, right?